TCS Daily


Climate Change: A Longer View

By Bob Carter - May 3, 2004 12:00 AM

Over the last two years, the scientific framework within which an assessment of climate change is made has undergone dramatic revision. Not all at once, but inch by inch, in increments of understanding.

First has come the rigorous external review of some of the key findings and recommendations of the Intergovernmental Panel on Climate Change (IPCC), as contained in its last "advice to governments" report in 2001. Review of the IPCC report has been accomplished mainly by free-thinking persons who are often referred to in derogatory terms as "climate skeptics" or "contrarians". In fact, those who have criticized the IPCC findings include many outstanding professional persons in their own right, nearly all of whom see themselves as climate "agnostics" rather than "skeptics". That is to say, they have no preconceived beliefs about the current pattern of climate change, nor any particular expectation for change to occur one way or the other. They are, however, united in their wish for the public to be informed of the main facts and conclusions about climate change free of any political agenda or IPCC intrigue.

Sadly, a foremost Australian member of this group died recently. John Daly, a retired Tasmanian engineer, maintained an eponymous web site devoted to critical analysis of climate change science. Together with another similar site maintained by Sherwood Idso, a senior meteorologist from Arizona, Daly's web site provided critical and dispassionate discussion of the torrent of frisbee-science which today masquerades as discussion of climate change in the daily media. These web sites, and other publications, have drawn attention to the increasing shakiness of the three main arguments which comprise the IPCC case for a human influence on climate change.

Three Arguments

Argument one asserts that ground-based temperature measurements have been corrected adequately for environmental effects, including especially the urban "heat island" effect, and that the pattern of global change in temperature which results -- about a 0.60 C increase over the last 100 years -- is likely to have a human cause. In actuality, that part of the claimed increase in temperature which occurred over the last 20 years is contradicted by two alternative measurements of atmospheric temperature made from weather balloons and satellites, the patterns of which agree with each other and show little or no long-term trend of temperature change. At the very least, this discrepancy casts doubt on the adequacy of the heat island correction which has been made to the records.

Argument two, after papers by statistician Michael Mann and co-authors, asserts that both the peak magnitude and the rate of temperature increase over the last 100 years are exceptional by comparison with the preceding 900 years. But recent published papers by other scientists have demolished this argument and shown that Mann's work is statistically unsound; both its historical analysis and its projected peak of warming at the recent turn of the century are now known to be flawed. And anyway, irrespective of recondite statistical arguments, many earlier published geological studies show that the rate and magnitude of climate change over historic times lies within the envelope of natural variation.

The third IPCC argument rests upon complex computer models which attempt to predict the rate of warming for the increasing rate of carbon dioxide in the atmosphere through to the year 2100. However, these models are unable to simulate 20th century climatic history accurately, and also fail when tested against the last 20 years of accurate data from satellites and weather balloons. A primary reason for the mismatches is probably that the computer models assume an unrealistically high temperature sensitivity for atmospheric greenhouse gas accumulation.

The flaws in these three IPCC arguments are cumulatively fatal. But, in addition, it has become increasingly apparent lately that the 1,000-year interval which is the context for most IPCC advice and analysis is a completely inadequate period over which to assess global climate change. The focus of discussion, therefore, is shifting away from the short-term mechanisms studied by meteorologists and climatologists, to attending more to the knowledge base for climate change which exists in the geological record over tens and hundreds of thousands of years.

Another Shift

Which brings us to the second, and quantum, shift in the recent climate debate. Bill Ruddiman, a geologist and climate scientist from the University of Virginia, has recently re-analyzed the history of atmospheric greenhouse gases as revealed by air bubbles trapped in polar ice cores from Greenland and Antarctica. In an important paper in the journal Climatic Change, Ruddiman shows that greenhouse gases reached a peak at the start of the present interglacial (the "Holocene") about 10,000 years ago and then, after starting to decline as anticipated from the natural solar cycle, commenced to rise again. Carbon dioxide shows an increase from about 8,000 years onwards, and methane from about 5,000 years onwards (Fig. A).

This pattern is at variance with the well-documented record of the three previous glacial/interglacial periods extending back to 350,000 years ago. During these earlier cycles, greenhouse gases declined steadily after reaching their immediate post-glacial (warm) peak, leading to the ensuing glaciation.

The unusual pattern of greenhouse gas history during the Holocene demands an explanation, and Ruddiman provides it. It is that the increases in methane and carbon dioxide occurred over the time when human agriculture and population increased during the Neolithic period of human culture and the subsequent development, after about 5,000 years ago, of organized civilizations. In Ruddiman's view, these human-caused greenhouse gas increases have already prevented Earth's climate from deteriorating into the next global glacial episode.

Deforestation, and the development of rice irrigation, grain cropping and animal husbandry, all act to increase the emission of methane and carbon dioxide beyond their "natural" level. When continued over 5,000-8,000 years, these human activities may have added approximately twice the amount of greenhouse gas to the atmosphere as has been contributed (at a much higher rate) during the short industrial era.

In a fascinating, though not completely convincing, aside, Ruddiman also argues that the minor decreases in carbon dioxide which occurred more than once during the mediaeval era are the imprint of episodes of plague, which led to huge mass mortality, the abandonment of settlements and the reversion of agricultural land to forest. Accordingly, he asserts that at these times carbon dioxide was sequestered from the atmosphere back into vegetation.

New Questions

The focus of the climate change debate has shifted irrevocably. Given Ruddiman's findings the key question now is not "is industrial-age, human-caused global warming occurring?", but rather "are we sure that the human effect on climate over the last 8,000 years has helped to prevent the occurrence of another glaciation?" Should the answer to that question be yes, then it prompts the further question: "do we wish to maintain the human warming effect, or instead to counteract it and allow Earth's climatic cycle to drop back into its next (natural) glacial episode?".

For people who live in continental Europe or northern North America, answering the last question may seem to be a no-brainer; such persons may have more to thank our distant ancestors for than they could ever have imagined. If Ruddiman is right, then the effect of human activity upon climate has already been profound, and the industrial age acceleration of greenhouse gas inputs to the atmosphere merely continues an existing and beneficial multi-millennial trend.

Though policy-makers are apparently itching to act "now" on climate change, it is clear that for the moment the status quo and "do no harm" is the preferred option. A much better understanding of natural cycles of climate change and of the magnitude of human effects on climate are needed before we consider implementing global mitigation measures. In such a regard, neither current IPCC advice nor the Kyoto Protocol are remotely adequate as a basis for action.

By encouraging the scientific angels to dance on the head of the "only-1,000-year-history" pin, the IPCC has so far signally failed to resolve the debate about a human influence on climate. It now needs to refocus its effort into understanding the causes of climate trends over a statistically more significant period, at the same time seeking to understand the unknown mechanisms which in the past have caused abrupt climate changes over periods as short as a decade.

Professor Bob Carter, of the Marine Geophysical Laboratory, James Cook University, is a former Director of the Australian Secretariat for the Ocean Drilling Program, the world's pre-eminent international collaborative program in environmental and geological science.


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